1. Field of the Invention
The present invention relates to a mobile communication system in which data is transmitted and received through a wireless transmission line between a network-side device (node) and a mobile station device, and to a mobile station device and a network-side device for use in the mobile communication system.
2. Description of the Related Art
As an error-correcting function in a data link layer, an “Automatic Repeat reQuest (ARQ)” system has been known.
Under the ARQ system, in a data link layer, a data block received from a higher layer is divided into data units of a predetermined size for transmission to a lower layer. Since retransmission processing by the ARQ system is performed on individual data units in the data link layer, a sequence number is given to each data unit to uniquely identify the data unit.
In a data link layer of a receiving-side device, the presence or absence of a reception error in a data unit identified by a sequence number is detected, and the result of the error detection is informed to a transmitting-side device as acknowledgement information or a retransmission request.
The “acknowledgement” is control information which indicates up to what sequence number data unit the receiving-side device has successfully received without interruption, indicating the sequence number of a data unit, among data units not yet received, to receive next.
The “retransmission request” is control information which indicates what sequence number data unit could not be received.
For example, the transmitting-side device adds information called a polling bit to a data unit, thereby indicating a trigger to issue the acknowledgement and the retransmission information to the receiving-side device. Generally, a trigger to issue the acknowledgement and a trigger to issue the retransmission request are the same.
A system in which only a data unit (data units) in which a reception error is detected can be selectively retransmitted using the acknowledgement and the retransmission request as described above is called a “Selective-Repeat ARQ system”.
In the Selective-Repeat ARQ system, a transmitting-side device and a receiving-side device are each provided with a buffer in which a certain number of data units can be held, so that the transmitting-side device can successively transmit a maximum number of N data units without receiving the acknowledgement.
FIG. 1 illustrates an example of operations in which data units are transmitted and received in a data link layer using a Selective-Repeat ARQ system of a related art.
In the example of FIG. 1, a network-side device (transmitting-side device) 7 can transmit up to three data units without receiving an acknowledgement from a mobile station device (receiving-side device) 5. A polling bit informed from the network-side device 7 is assumed as a trigger to issue the acknowledgement.
In FIG. 1, “SN” represents a sequence number uniquely identifying a data unit; “ACK” represents an acknowledgement; “NACK” represents a retransmission request; and “P” represents a polling bit.
In the example of FIG. 1, a polling bit P is added when the last data unit in a transmission buffer is sent to a lower layer, or added to a data unit including the end of a data block in a higher layer, or added to the last data unit in a retransmission buffer when retransmission processing is performed. Here, one data block is made up of three data units.
In the data link layer, three buffers are provided: the transmission buffer, the acknowledgement waiting buffer, and the retransmission buffer.
Here, the transmission buffer divides a data block in a higher layer into data units, and holds them until actually sending them to a lower layer.
The acknowledgement waiting buffer holds data units sent to the lower layer until receiving an acknowledgement, and has a specified size capable of holding N data units. The N is called a window size.
The retransmission buffer holds, upon receipt of a retransmission request, a data unit until sending it to the lower layer.
The example of FIG. 1 shows the process in which a reception error occurs in a data unit of SN=4, and the data unit of SN=4 is retransmitted.
FIG. 2 illustrates multiple retransmissions which can occur in a radio access network having a long transmission delay when the related-art retransmission processing in a data link layer is adopted.
In the example of FIG. 2, a reception error occurs in a data unit of SN=1, and the network-side device 7 performs retransmission processing on the data unit of SN=1 in response to a first piece of retransmission request, and then performs retransmission processing on the data unit of SN=1 for a second time in response to a second piece of retransmission request (which reached to the transmitting-side device) before the result of reception of the retransmitted data unit is reflected in the mobile station device 5 (the receiving-side device).
In the example of FIG. 2, a polling bit P is added when the last data unit in the transmission buffer is sent to the lower layer, or added to a data unit including the end of a data block in the higher layer, or added to the last data unit in the retransmission buffer when retransmission processing is performed. Here, one data block is made up of three data units.
As shown in FIG. 3, a general mobile communication system includes mobile station devices 5, radio base station devices 6, a network-side device 7, wireless transmission lines 2 connecting the mobile station devices 5 and the radio base station devices 6, and wired transmission lines 4 connecting the radio base station devices 6 and the network-side device 7.
Each radio base station devices 6 accommodates a plurality of mobile station devices 5 in a service area 3 through wireless transmission lines 2.
The network-side device 7 accommodates a plurality of radio base station devices 6 through the wired transmission lines 4, and is intensively provided with functions required to accommodate the mobile station devices 5.
Here, transmission line characteristics of the wireless transmission lines 2 depend on buildings or the like, varying from time to time with the movements of the mobile station devices 5. Thus the wireless transmission lines 2 have transmission error rates higher than those of the wired transmission lines 4.
Therefore, it is difficult for the wireless transmission lines 2 to ensure communication quality compared to the wired transmission lines 4, and retransmission processing using an ARQ system is required in the above-described mobile communication system.
Also, in this mobile communication system, a transmission delay is long, because a transmission line between the network-side device 7 and each mobile station device 5 (a radio access network 1 includes a wireless transmission line 2 and a wired transmission line 4) passes through the radio base station device 6 which performs modulation and demodulation in the wireless transmission line 2.
In efforts to increase a transmission rate of data units in the radio access network 1 characterized by such high transmission error rates and long time delays, the following two problems will arise:
The first problem is that in a transmitting-side device, as a result of increasing the transmission rate of data units, successively transmitted data units can reach the upper limit N of the acknowledgement waiting buffer (window size) before an acknowledgement transmitted from a receiving-side device reaches the transmitting-side device.
The second problem is that multiple retransmissions can occur, because intervals at which to issue the control information (a retransmission request) at a receiving-side device are shorter than a period of time between the time at which the retransmission request is issued at the receiving-side device and the time at which the retransmitted data unit reaches the receiving-side device.
Here, the “multiple retransmissions” indicate a phenomenon in which retransmission processing is performed several times on a data unit at the transmitting-side device before an acknowledgement issued at the receiving-side device on the data unit retransmitted from the transmitting-side device reaches the transmitting-side device.
The above two problems have characteristics that the longer the transmission delay between the mobile station device 5 and the network-side device 7 is, the lower the effective transmission rate of data units is.
To lessen the first problem, it is necessary to increase the issue rate of the control information (the acknowledgement and the retransmission request).
However, this causes frequent multiple retransmissions which are the second problem, resulting in wasteful consumption of the transmission bandwidth between the mobile station device 5 and the network-side device 7.
Contrarily, when the issue rate of the control information (the acknowledgement and the retransmission request) is reduced to reduce multiple retransmissions, the first problem worsens.
Hereinafter, as an example of a mobile communication system, retransmission processing in an IMT-2000 system will be described.
The IMT-2000 system provides a method of putting a limit on the issue rate of the control information so as to efficiently use the transmission bandwidth of wireless transmission lines, in addition to the above-described general retransmission processing in a data link layer.
The method of limiting the issue rate of the control information is implemented by setting a control information issue prohibition period which is fixed.
At the point of time when a trigger to issue the control information is obtained, the following processing is performed. That is, if the control information issue prohibition period has not elapsed since the last issue of control information, the issue of new control information is suspended.
At the point of time when the control information prohibition period has elapsed since the last issue of control information, new control information is issued.
The IMT-2000 system also proposes a means for adjusting the issue of a retransmission request to delay in a transmission line, which is called the “Estimated PDU Counter (EPC)”.
When the retransmission request is issued at a receiving-side device, the EPC calculates, as a “control information issue prohibition period which is adapted to the current bit rate”, a reasonable period between the execution of retransmission processing on a data unit at a transmitting-side device in response to the retransmission request and the completion of reception of the retransmitted data unit at the receiving-side device.
During the control information issue prohibition period, the receiving-side device terminates the issue of new control information even when a trigger to issue next control information occurs.
That is, when getting a trigger to issue the control information during the control information issue prohibition period in the EPC, the receiving-side device generates and issues the control information after a lapse of the control information issue prohibition period.
The use of the EPC can also prevent occurrence of the multiple retransmissions as described above. However, with the EPC, when once a retransmission request is issued, even issue of different retransmission requests on different data units and an acknowledgement is also prohibited. Thus, the above-described first problem still cannot be solved.
As described above, the Selective-Repeat ARQ system used in a data link layer implements an error-correcting function using two pieces of control information, an acknowledgement and a retransmission request.
However, in order to increase a transmission rate of data units in the radio access network 1 characterized by a long transmission delay, it is required for a receiving-side device to issue an acknowledgement with high frequency due to the limitation of the size (window size) of acknowledgement waiting buffers provided in the mobile station devices 5 and the network-side device 7.
Generally, the acknowledgement and the retransmission request are handled as the same kind of control information, and are communicated to a transmitting-side device based on the same trigger for issue.
Therefore, the issue of an acknowledgement with high frequency also triggers simultaneous issue of a retransmission request with the same frequency.
As a result, in the radio access network 1 characterized by a high transmission error rate and a long transmission delay, a retransmission request including the same contents is issued to a transmitting-side device, causing the problem of multiple retransmissions.